Patent U.S. Pat. No. 6,386,458 describes a data storage medium comprising an information carrier which is rolled up in a spiral fashion and on which written information patterns are provided which can be read optically, due to local refractive index variations. This data storage medium comprises a plurality of information layers, which are read from the inside by means of an optical beam arranged in the inside of the winding. Such a data storage medium is compact and can, in theory, comprise a large number of information layers.
However, the number of information layers in such an information carrier is limited. First, because the luminous intensity of the optical beam decreases with each additional layer crossed. Actually, when the optical beam has to pass many layers for interacting with a layer, interaction takes also place in the layers that are not read out, reducing the intensity of the optical beam. Additionally, the local refractive index variations of the written information patterns in the layers that are not read out cause refraction and scattering of the traversing light-beam, leading to deteriorated writing and reading.
Hence, such a rolled up information carrier is not suitable for multi-layer information carriers, in particular for information carriers comprising more than three layers.